It is well known that vehicles in general, and heavy vehicles in particular, may become stuck when trying to move from stationary in a situation of unfavorable running surface conditions. Such unfavorable running surface conditions may take the form of, for example, snow or other soft running surfaces such as sand, earth etc.
When a vehicle has to move off in such conditions it is usually in a situation where one or more powered wheels have become stuck in a cavity or a hollow. In that case, the engine torque (driving force) applied to set the vehicle in motion again may cause the powered wheels to slip before they have moved up out of the cavity/hollow. This may result in the cavity/hollow becoming still deeper and hence still more difficult to drive out.
When such situations arise, one or more “proven” methods for trying to free the vehicle are often used. For example, the vehicle's air suspension system may be used and/or tag axles be raised to increase the load on the powered wheels and thereby make it easier for them to gain a grip on the running surface. Another common method often used in the case of just such cavities/hollows is a rocking procedure, wherein the driver tries to rock the vehicle free by increasing the force applied to the powered wheels, preferably with a relatively high gear engaged, until the wheels spin free, whereupon the driver releases the accelerator pedal and disengages the clutch so that the vehicle rolls back. This procedure is then repeated when the ongoing movement in the opposite direction has ceased. Continuing to rock the vehicle in this way may ultimately achieve sufficient force to take the vehicle up out of the cavity/hollow.
As heavy vehicles have a high available torque even when idling, the rocking function can often be carried out by using only the clutch, whereby driving force is applied to the powered wheels by the driver releasing the clutch pedal and then depressing it quickly when the powered wheels begin to spin, so that the vehicle can roll back down into, and up on the opposite side of, the cavity/hollow. Once the movement in the opposite direction has ceased, the clutch can again be used to apply driving force to the powered wheels when the vehicle again begins to move in the initial direction, in order thereby to try to reach a higher speed than in the previous attempt to drive up out of the cavity/hollow. The procedure (the rocking) is then repeated until the vehicle has freed itself from the cavity/hollow or a need for further assistance is established.
It has become increasingly common for heavy vehicles to be fitted with automatically operated gearboxes. These gearboxes do not however usually take the form of automatic gearboxes in the traditional sense, but rather of “manual” gearboxes in which gear changing is controlled by a control system, in part because manual gearboxes are substantially cheaper to produce, but also because they are more efficient.
In one type the driver uses a clutch pedal to set the vehicle in motion from stationary, but all other gear changing can be effected by the vehicle's control system without involving the clutch at all. Instead the gear changes are carried out “torque-free”, i.e. the torque delivered from the engine is adjusted to a suitable level to reduce the torque transmitted at the engagement points of the relevant gears. With power trains of this type, the driver therefore has access to a clutch pedal which he/she can also use to carry out the rocking function as above.
It is however also common for vehicles with a gearbox of the above type to be equipped with a fully automatically controlled clutch, whereby the driver has access to only an accelerator pedal and a brake pedal. When moving off as normal in a vehicle equipped with an automatically controlled clutch, the vehicle's control system will fully close the clutch when the driver presses the accelerator pedal. Thereafter the clutch will not open until the driver presses the brake and the engine speed drops to idling speed. This has the disadvantage that, in a rocking procedure, the engine's moment of inertia, until the clutch opens, will brake the vehicle's backward roll, thus reducing the effect of the rocking function and, in the worst case, preventing the vehicle from successfully moving off.
There is therefore, at least in certain situations, a need for an improved method for setting in motion vehicles with automatically controlled clutches in cases where the vehicle has become stuck.